1. Field of the Invention
The present invention relates to an electronic musical instrument which creates a musical tone through use of a tone filter for a primary waveform signal abundantly containing harmonics.
2. Description of the Prior Art
Conventionally, a musical tone generating system for electronic musical instrument, represented by an electronic organ, a synthesizer and so forth, is roughly divided into an analog and a digital tone generating system. The digital tone generating system permits the synthesization of musical waveform elements by digital operations and has possibilities of creating timbre over a wide range. But the digital tone generating system has the defects of an enormous circuit arrangement and limitations on the sound synthesis owing to restrictions on the amount of operation and the operation time, and have been employed only in some high grade models. The analog tone generating system is comprised of a primary waveform generator which generates a primary waveform signal corresponding to a musical frequency and abundantly containing harmonic components, a filter circuit for controlling the harmonic components of the primary waveform signal in accordance with timbre desired to create and an envelope circuit for providing a desired envelope. These circuit elements are well-known VCO (Voltage-Controlled Oscillator), VCF (Voltage-Controlled Filter) and VCA (Voltage-Controlled Amplifier) which use voltage as a common control parameter, and have undergone various improvements. Also in connection with the analog tone generating system, many problems have been pointed out. For instance, the tone generating circuit has not markedly been improved as compared with other circuit elements of the musical instruments. Conventional tone generating circuits are mostly liable to produce what is called a "characteristic tone" which has a specific harmonic structure, and the "character" cannot be removed however intensively the tone is subjected to filtering by a tone filter. The reason for this is that since frequencies handled by the tone generating circuit are note frequencies which vary with the playing status, is is very difficult to form a predetermined primary waveform signal at any of the frequencies, which has been effected mostly by a duty ratio changing method through use of an even-order frequency divider. It has also been proposed to set the duty ratio or generate a pulse train by the employment of a shift register, ring counter, Johnson counter or the like. But these prior art methods are within the scope of an integral multiple of a basic clock signal, and are unable to completely eliminate the "character". Even if a circuit for producing a certain primary waveform signal should be implemented through utilization of complex and elaborate techniques, it would encounter the problems that in the case of creating a plurality of tones, no variations cannot be produced and that if many tones are produced, their tonal quality will be degraded.